There are technologies known in the related art for updating map data used in car navigation apparatuses by dividing a map into a plurality of units, creating map data files in correspondence to the individual units and individually updating the map data files (see, for instance, patent reference literature 1). There are also technologies in the related art which are adopted in map information services provided through computer communication, whereby only map data that are needed are transmitted to a map data display terminal without transmitting any map data already present in a display memory at the map data display terminal (see, for instance, patent reference literature 2).    Patent reference literature 1: Japanese Laid Open Patent Publication No. 2001-56823    Patent reference literature 2: Japanese Laid Open Patent Publication No. H11-295084